Solder pads and method of making a solder pad

ABSTRACT

A device including a first solder pad and a second solder pad comprised of a post-soldering alloy composition on a substrate is provided. The alloy composition comprises two or more elements, and the post soldering alloy composition of the first solder pad has different amounts of the two or more elements than the alloy composition of the second solder pad. A method of making a solder pad comprises masking a substrate comprising at least a first solder pad and a second solder pad, wherein the mask exposes a greater area of the first solder pad so that the deposited element becomes part of an alloy composition of the first solder pad upon soldering thereby changing the melting point of the first solder pad.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/285,907 filed Apr. 23, 2001, now abandoned, such applicationbeing incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to solder pads and methods of makingsolder pads, and in particular to solder pads comprising initial layeredstructures of varying composition to provide solder pads of differingsoldering temperatures.

BACKGROUND OF THE INVENTION

Manufacture of electronic assemblies often makes use of a techniquecalled “step-soldering.” In step-soldering, solder alloys of decreasingsoldering temperature are used to bond a die to a substrate, a substrateto a package, and a package to a housing or printed wiring board. Whenmade in a step soldering process, solder bonds made in a prior operationdo not reflow when subsequent operations are performed at step by stepdecreasing temperature decrements. The operation that is performedfirst, generally die to substrate bonding, uses a solder that has asoldering temperature that is higher than that of any solder used in asubsequent operation. Consequently, subsequent soldering operations donot disturb the previously soldered components.

In microelectronics and optoelectronics manufacturing, solder alloys aregenerally gold-based, like gold-tin and gold-germanium, to provide ahigh soldering temperature and to provide compatibility with gold andother precious-metal metallization used for electrical contact between asemiconductor die and substrate.

Solder pads are applied by depositing alternating layers of the desiredfinal alloy constituents in separate layers, for example: gold, tin,gold, tin, gold, tin with eleven layers total not being unusual. Thelayered structure is heated, and, upon reaching a soldering temperaturewhich is above the melting point of tin, the metals inter-diffuse toform the desired solder alloy. Note that a dwell time at a temperaturebelow melting may also be used to achieve inter-diffusion of the layers.For example, inter-diffusion is essentially complete after one second at210° C. In a step-solder process, each subsequent step occurs at asoldering temperature lower than the previous soldering step so as notto remelt previous solder connections. To be robust in a manufacturingenvironment, there should be a large difference in the solderingtemperature between one solder step and the next.

From a practical perspective, step-soldering is limited to about threeor four steps between the first solder used and the last. The lastsolder used is typically a tin-lead solder having a low solderingtemperature. For this reason, conventional step soldering has difficultyaccommodating the mounting of multiple dies on a substrate in multistepmounting operations. It would be desirable to be able to precision placea die, like a gallium arsenide laser, on a substrate such as silicon, tosolder it into position, to move the substrate, and then to repeat theoperation to precision mount another die, like a photodetector chip.Thus, a need exists for a configuration of solder pads which facilitatesthe mounting of multiple components in a multi-step process.

SUMMARY

In accordance with the present invention, a device is provided having atleast a first solder pad and a second solder pad each disposed on asubstrate. The first and second solder pads each comprise a plurality oflayers of elements. The elements are selected from those that formsolder alloys. The second solder pad has first layer that has a volumewhich is less than the volume of a second layer of the second solderpad. Upon heating the solder pads, a plurality of layers alloy to form apost-soldering alloy composition. After heating, the device includes afirst solder pad, comprised of a first post-soldering alloy composition,disposed on a substrate. The device also includes a second solder pad,comprised of a second post-soldering alloy composition, disposed on thesubstrate. The alloys comprise at least two elements. Further, the firstpost-soldering alloy composition has different amounts of the at leasttwo elements than the second post-soldering alloy composition.

A method in accordance with the present invention is provided for makinga solder pad comprising the steps of providing a substrate having afirst and second solder pads, and depositing on at least the firstsolder pad an element that becomes part of an alloy composition uponsoldering. The step of depositing an element provides a greater amountof the element on the first solder pad than on the second solder pad.The method also provides that the resulting alloy composition comprisesat least two elements so that the alloy composition of the first solderpad has a different ratio of the two elements than the alloy compositionof the second solder pad. The difference in ratios between the first andsecond solder pads provides a different soldering temperature of thefirst solder pad versus the second solder pad. The step of depositingmay also include the step of masking the substrate whereby the maskexposes a greater are of the first solder pad than the second solderpad. In such a case, the step of depositing also comprises depositingthe element through the mask on the first solder pad so that an area ofthe first solder pad covered with the element is greater than the areaof the second solder pad covered with the element.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of thepreferred embodiments of the present invention will be best understoodwhen read in conjunction with the appended drawings, in which:

FIG. 1 schematically illustrates a side elevational view of solder padson a substrate according to an embodiment of the invention.

FIG. 2 schematically illustrates a side elevational view of solder padson a substrate according to a further embodiment of the invention.

FIG. 3 shows a gold-tin phase diagram;

FIG. 4 shows a gold-germanium phase diagram;

FIG. 5 shows a gold-silicon phase diagram;

FIG. 6 illustrates a flow chart of a method of the present invention;and

FIG. 7 schematically illustrates solder pads comprising a post-solderingalloy composition.

DETAILED DESCRIPTION

Referring to FIG. 1, solder pads 113 whose melting points differaccording to the ratio of the materials making up the solder pads 113are provided. This difference in melting points is conveniently effectedby varying the area of a deposited layer of material 17 on the solderpads 113. As an illustrative example, a layer of a first metal, such astin, may be deposited by physical vapor deposition through maskapertures onto solder pads which comprise a second metal, such as gold.The mask used to deposit the tin comprises apertures of different sizesso that first solder pad receives a different amount of tin than asecond solder pad. Since the melting point of the layered solder paddepends on the ratio of the materials in the solder pad, varying theamount of tin deposited on the solder pads provides solder pads havingdifferent melting points.

Referring now to FIG. 3, which shows a gold-tin phase diagram, themelting temperature of a solder pad comprising multiple layers of goldand tin can be determined based on the ratio of total gold to total tin.For example, a solder pad formed from melting layers of told and tin toform an alloy comprising 20 wt. % total tin yields a eutecticcomposition 30 having a melting point of 278° C. The melting points ofnon-eutectic compositions are also shown in FIG. 3. The liquidus lines32, 34 indicate the temperature at which a non-eutectic composition willbecome fully liquid. In FIG. 4, the gold-germanium system provides aeutectic composition 40 and liquidus lines 42, 44. Likewise, thegold-silicon system shown in FIG. 5, provides a eutectic composition 50and liquidus lines 52, 54. Thus, for each of these three binary goldsystems, varying the percentage of gold in the composition effectsvariation among the melting temperatures of the resulting alloys. Forexample, as shown in FIG. 3, the composition 90 wt. % gold/10 wt. % tinhas a melting point 36 of about 740° C. Thus, changing the compositionfrom 20 wt. % tin to 10 wt. % tin produces a change in the meltingtemperature of the resulting alloys from 278° C. to 740 ° C.

Turning now to FIG. 1, an illustrative embodiment of the invention isshown. A substrate 10 comprises gold metallization 12 disposed on asilicon wafer 11. Solder pad bases 13, 14, 15 comprising alternatinglayers of gold and tin are disposed on the gold metallization 12effecting electrical contact with the gold metallization 12. The solderpad bases 13, 14, 15 may be formed by any process which provides preciselocation of solder pad bases and permits the formation of alternatinglayers of elements that can alloy to form a solder, such as gold andtin. It is further desirable that the processes used to deposit thesolder pad bases 13, 14, 15 provide sufficient flexibility so that thenumber of layers and relative thickness of the layers can be controlled.Such processes can include sputtering, evaporating, screening, vapordepositing, or plating with the optional use of a mask andphotolithography techniques. In one aspect of the invention, lift-off,dry mask, and etching are among the techniques that may be used. Inaddition the layers can be created by any combination of theseprocesses.

For example, the solder pad bases 13, 14, 15 may be formed by physicalvapor deposition through a mask to provide the solder pad bases 13, 14,15 at desired locations on the gold metallization 12. Alternatively, thesolder pad bases 13, 14, 15 may be formed from a continuous sheet ofalternating layers of tin and gold disposed on the gold metallization12. The solder pad bases 13, 14, 15 are formed by removing portions ofthe tin and gold sheet using masking techniques to define the locationsof the solder pad bases 13, 14, 15, followed by etching, milling, orother removal processes to remove the exposed sheet portions.

Additional layers of tin and/or gold are deposited on at least one ofthe solder pad bases 13, 14, 15 so that the gold-tin composition differsamong the completed solder pads 113, 114, 115. For example, a mask 16having apertures 19 a and aperture 19 b may be provided to permitdeposition of gold and/or tin to selected portions of the solder padbases 13, 14, 15. In particular, as shown in FIG. 1, the mask 16comprises apertures 19 a and aperture 19 b of differing cross-sectionwhich are disposed in registry with the solder pad bases 14 and 15,respectively. In addition, the mask 16 occludes the solder pad base 13,preventing further deposition of material on the solder pad base 13.Thus, the completed solder pad 113 is the same as solder pad base 13.

The apertures 19 a have a cross-section area which defines a portion ofthe solder pad base 14 to receive additional deposited material. Asillustrated in FIG. 1, the combined cross-sectional areas of apertures19 a provide for metallization of 50% of the surface area of solder padbase 14. Aperture 19 b, which is disposed in registry with solder padbase 15, has cross-sectional area equivalent to the surface area ofsolder pad base 15, and therefore permits metallization of 100% of thesurface area of solder pad base 15.

With the mask 16 in position, any number of deposition steps may beperformed to create further layered structures on the solder pad bases14, 15. For example, as depicted in FIG. 1, an additional layer of tin17 and gold 18 may be disposed on the solder pad bases 14, 15. Since thetin layer 17 and gold layer 18 are deposited during the same steps, thethickness of these layers on solder pad base 14 and solder pad base 15is equal. However, due to the different cross-sectional areas of theapertures 19 a as compared to aperture 19 b, the added layers of tin 17and gold 18 provide differing amounts of tin and gold on solder pad base14 versus solder pad base 15. Alternatively, the depositions effectedthrough apertures 19 a may be performed at a different step than thedepositions through aperture 19 b so that the thickness of the layers isdifferent on solder pad base 14 relative to solder pad base 15. The tinand gold layer 17, 18 may each be of different thickness than the layerspreviously deposited to form the solder pad bases 13, 14, 15. Byproviding a ratio of tin to gold in the layers 17, 18 different from theratio of tin to gold in the solder pad bases 14, 15, the melting pointsof the solder pads 114, 115 can be differentiated by virtue of thediffering overall composition of the solder pads 114, 115.

However, it should be noted that even when solder pads have the sameratio of total tin to gold, such as solder pad 113 and solder pad 115,the melting points of such solder pads can be different due to thediffusion of gold from a metallization layer 12 into the solder padsduring the melting of the layered solder pads to form an alloy. Themelting point differentiation is possible due to the fact that solderpad 115 has a greater total amount or volume of gold and tin than solderpad 113. The increased amount of tin and gold in solder pad 115 reducesthe percentage contribution of gold metallization 12 diffusion into thesolder pad 115. Thus, while the layered structures of solder pads 113,115 may each comprise the same composition, the diffusion of gold fromthe gold metallization 12 into solder pad 113 during melting introducesa greater percentage of gold into solder pad 113. Such diffusion canalter the melting temperature of solder pad 113 to a greater extent thanthe melting temperature of solder pad 115.

While the layers 17, 18 are shown in FIG. 1 as the final layers appliedto the solder pad bases 14, 15, the layers of varying area or volumeneed not be the last layers deposited. Moreover, additionaldifferentiation among solder pad compositions may be achieved bydeposition through additional masks having apertures which introduceadditional variation in the amount of material deposited on a solderpad. In addition, although the layers of gold and tin are shown to haveapproximately equal height in FIG. 1, additional control of solder padcompositions may be effected through application of layers havingunequal height and therefore unequal volume. For example, in certainapplications it may be desirable that each solder pad comprises the sametotal volume so that each solder pad will have the same height. In suchcases it may be convenient to do so by applying layers of unequalheight. Moreover, it may be desirable to control the speed with whichthe layers inter-diffuse to form an alloy. Providing layers ofrelatively reduced height can decrease the diffusion distance betweenlayers and therefore decrease the time to form a homogeneous alloy.

FIG. 2 shows another illustrative embodiment of the invention. Asubstrate 20 comprising a silicon wafer 21 and gold metallization 22 hassolder pad bases 23, 24, 25 comprising alternating layers of gold andtin deposited in a prior operation. The solder pad bases 23, 24, 25 maybe deposited by any of the methods described above with respect tosolder pad bases 13, 14, 15 of FIG. 1. As shown in FIG. 2, a mask 26 isprovided having differently sized apertures 29 a, 29 b which are placedin registry with the solder pad bases 24, 25, respectively. The mask 26allows no further metallization of solder pad base 23, furthermetallization of 50% of the surface area of solder pad base 24 throughaperture 29 a, and metallization of 100% of the surface area of solderpad base 25 through aperture 29 b.

An additional layer of metal, such as tin layer 27, is deposited on tothe solder pad bases 24, 25 through the apertures 29 a, 29 b,respectively. The tin layer 27 may be of different thickness than thelayers previously deposited to create the solder pad bases 23, 24, and25. By adjusting the thickness of the additional tin layer 27 appliedthrough differently sized apertures 29 a, 29 b, the melting points ofthe solder pads 124, 125 can be adjusted in a similar manner to thatdescribed above with respect to the solder pads 114, 115 of FIG. 1.

Thus, as shown in the embodiments of FIGS. 1 and 2, a configuration ofsolder pads is provided that is suited to multiple-step, step-solderingprocesses. Any number of solder pads may be provided on a substrate formaking electrical connection to device components. Each of the solderpads, or individual group of solder pads, may have a separate meltingpoint provided by differences in the layered structure of the solderpads. For each separate melting point provided, a separate step of thestep-soldering process can be performed.

Step-soldering using the above described embodiments is performed byheating the substrate to a first temperature corresponding to themelting point of a first group of solder pads. The first temperature isgenerally selected to be the highest melting temperature of any of thesolder pads 113, 114, 115. The layers of material in the first group ofsolder pads, as well as any solder pads having a lower meltingtemperature, alloy to form substantially homogeneous compositions. Forexample, the solder pads 113, 114, 115 shown in FIG. 1 after meltingform alloyed solder pads 33, 34, 35, respectively, as shown in FIG. 7.

Electrical connection is then made between any desired components andthe molten first group of solder pads, and the first group of solderpads is allowed to cool and solidify. Subsequent connections are made byheating the substrate to a second temperature lower than the firsttemperature so that the solder pads of the first group do not reflow.The second temperature is selected so that a second group of solder padsis brought to the soldering point so that electrical connection can bemade between additional device components and the second group of solderpads. Additional steps of the step-soldering process continue where eachsubsequent temperature used to melt additional solder pads is lower thanany previous temperature, so that the previously soldered solder pads donot reflow. The composite layered structure provides control of thesoldering temperature of a solder pad by controlling the number oflayers and area of coverage of material provided in a layer of thesolder pad. The tuning of melting temperatures in this manner permitsmany different solder temperatures to be defined among the solder pads,permitting an equivalent number of discrete steps in the step-solderingprocess. Note that, in another embodiment of the invention, when thesolder pads are on a gold metallized substrate, the reflow temperatureof a first solder pad can actually increase over the original meltingtemperature. This is because gold from the metallization can diffuseinto the first pad, raising the gold content of the first solder alloy,and hence, it's reflow temperature. This allows other solder pads, suchas second solder pads with greater tin content, or third solder pads notdeposited on gold metallization, to be reflowed at the same temperatureas the first solder pad, without causing reflow of the first solder pad.

The invention also provides a method for creating the solder pads. Asillustrated in FIG. 6, the method begins by providing a substrate, atstep 300, on which solder pads may be formed. A first layer of firstsolder forming material is deposited on the substrate at desiredlocations of the solder pads, at step 310, to provide a first layer ofthe solder pad base. The first layer may be deposited using maskingtechniques, where the apertures in the mask define the location and sizeof the first layer of the first solder forming material. For example,the deposition of the first layer may be performed by physical vapordeposition through apertures of the mask provided at step 305.Alternatively, deposition of the first layer may be performed in severalstages, where the first stage applied a layer of solder forming materialon the substrate; the second stage provides a mask, at step 305, to maskoff the areas where the first layer is to remain; and, the third stageremoves the layer of solder forming material exposed by the mask, todefine the locations of the solder pads. In addition, photolithographicprocesses may be used to form the solder pads.

The method further comprises optionally providing on the solder pad baseone or more additional layers of solder forming material capable offorming an alloy composition with the first solder forming material, atstep 320. The optional solder forming materials may be different from,or the same as, the first solder forming material. These optional layersmay be provided to some or all of the solder pad bases and cover all orpart of the upper surface of any such solder pad base to which suchoptional layers are applied. The optional layers may be provided usingany of the techniques described above with regard to providing the firstlayer at step 310, including the use of a mask.

In addition, the method comprises providing one or more tuning layers toat least one solder pad base, at step 330. Each tuning layer comprises asolder forming material capable of forming an alloy composition with thesolder forming materials previously applied to the respective solder padbase. The tuning layer is deposited in such a manner so as to provide atleast two solder pad bases having differing amounts of the materialdeposited in the tuning layer. For example, the tuning layer may bedeposited through a mask, provided at step 325, having apertures ofdifferent size. The mask apertures are registered to the solder padbases at step 325. Alternatively, the tuning layer may be applied todifferent solder pads at different steps so that the thickness of thetuning layer varies among selected solder pads.

Finally, the method includes, at step 340, providing on the solder padbase one or more optional layers of solder forming material capable offorming an alloy with the solder forming materials previously applied tothe solder pad bases. These optional layers may be provided to some orall of the solder pad bases and cover all or a part of the upper surfaceof any such solder pad base to which such optional layers are applied.The optional layers may be provided using any of the techniquesdescribed above with regard to providing the first layer at step 310,including the use of a mask. Accordingly, the method provides solderpads of varying composition with corresponding varying melting points.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it will be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. Forexample, while the above embodiments comprise solder pads having abinary composition, solder pads comprising three or more materials mayalso be created. It should therefore be understood that this inventionis not limited to the particular embodiments described herein, but isintended to include all changes and modifications that are within thescope and spirit of the invention as set forth in the claims.

1. A method of making a solder pad comprising: masking a substratecomprising at least a first solder pad and a second solder pad with amask, wherein the solder pads each contain an alloy composition uponsoldering having at least two of the same elements and wherein the maskexposes a greater area of the first solder pad than the second solderpad; and depositing on at least the first solder pad at least one of theelements that becomes part of the alloy composition upon soldering,wherein the resulting alloy composition of the first solder pad has adifferent ratio of the two elements by weight than the alloy compositionof the second solder pad.
 2. The method of claim 1, wherein the alloycomposition comprises gold-tin.
 3. The method of claim 1, wherein thealloy composition comprises gold-germanium.
 4. The method of claim 1,wherein the elements are selected from the group consisting of gold,tin, germanium, silicon and mixtures thereof.
 5. The method of claim 1,wherein the depositing the element comprises depositing a greater amountof the element on the first solder pad than on the second solder pad. 6.The method of claim 1, wherein the masking a substrate comprisesoccluding the second solder pad.
 7. The method of claim 1, comprisingthe heating the substrate to a temperature sufficient to cause all thesolder pads to melt to form solder alloys.
 8. The method of claim 7comprising the maintaining the substrate at a selected temperaturesufficient to cause all of the solder pads to be melted and solderingselected components to the substrate at selected solder pads.
 9. Themethod of claim 8 comprising the steps of reheating the substrate toanother temperature sufficient to cause only some of the solder pads tomelt but not solder pads to which components have been soldered so thatselected components may be soldered to the substrate at selected ones ofthe melted solder pads.
 10. The method according to claim 1, wherein thedepositing an element comprises depositing the element through the maskso that the first solder pad has a different ratio of the elementdeposited through the mask than that of the second solder pad.
 11. Amethod of making a solder pad comprising: providing a substratecomprising at least first and second solder pads, the solder pads eachcontaining an alloy composition upon soldering having at least two ofthe same elements; depositing on at least the first solder pad at leastone of the elements that becomes part of the alloy composition uponsoldering, wherein the amount of the element deposited on the firstsolder pad covers a greater fraction of the area of the first solder padthan covers the fraction of the area of the second solder pad, andwherein the resulting alloy composition of the first solder pad has adifferent ratio by weight of the two elements than the alloy compositionof the second solder pad to provide a different soldering temperature ofthe first solder pad relative to the second solder pad.
 12. The methodof claim 11, wherein the alloy composition comprises gold-tin.
 13. Themethod of claim 11, wherein the ahoy composition comprisesgold-germanium.
 14. The method of claim 11, wherein the elements areselected from the group consisting of gold, tin, germanium, silicon andmixtures thereof.
 15. The method of claim 11, wherein the depositing theelement comprises depositing a greater amount of the element on thefirst solder pad than on the second solder pad.
 16. The method of claim11, wherein the masking a substrate comprises occluding the secondsolder pad.
 17. The method of claim 11 comprising the heating thesubstrate to a temperature sufficient to cause all the solder pads tomelt to form solder alloys.
 18. The method of claim 17 comprising themaintaining the substrate at a selected temperature sufficient to causeall of the solder pads to be melted and soldering selected components tothe substrate at selected solder pads.
 19. The method of claim 18comprising the reheating the substrate to another temperature sufficientto cause only some of the solder pads to melt but not solder pads towhich components have been soldered so that selected components may besoldered to the substrate at selected ones of the melted solder pads.20. The method according to claim 11, wherein the depositing an elementcomprises masking the substrate and depositing a first element of the atleast two elements through the mask so that the first solder pad has adifferent ratio by weight of the first element to the second element ofthe at least two elements than that of the second solder pad.
 21. Amethod of making a solder pad comprising: masking a substrate comprisingat least a first solder pad and a second solder pad, wherein the maskexposes a greater area of the first solder pad than the second solderpad; and depositing an element that becomes part of an alloy compositionupon soldering, wherein the resulting alloy composition comprises atleast two elements and wherein the alloy composition of the first padhas a different weight percentage of the element than the alloycomposition of the second pad.
 22. The method of claim 21 wherein thealloy composition comprises gold-tin.
 23. The method of claim 21 whereinthe alloy composition comprises gold-germanium.